Microemulsions are stable isotropic mixtures of oil, water, and surfactant which form spontaneously upon contact of the ingredients. Other components, such as salt or co-surfactant (such as an alcohol, amine, or other amphiphilic molecule) may also be part of the microemulsion formulation. The oil and water reside in distinct domains separated by an interfacial layer rich in surfactant. Because the domains of oil or water are so small, microemulsions appear visually transparent or translucent. Unlike emulsions, microemulsions are equilibrium phases.
Microemulsions can have several microstructures, depending upon composition and sometimes temperature and pressure. There are three most common structures. One is an oil-in-water microemulsion in which oil is contained inside distinct domains in a continuous water-rich domain. The second is a water-in-oil microemulsion, in which water is contained inside distinct domains (droplets) in a continuous oil-rich domain. The third is a bicontinuous microemulsions in which there are sample-spanning intertwined paths of both oil and water, separated from each other by the surfactant-rich film.
Polymerization of emulsified and microemulsified unsaturated hydrocarbon monomers is known, where high reaction rates, high conversions and high molecular weights can be achieved. A microemulsion can be distinguished from a conventional emulsion by its optical clarity, low viscosity, small domain size, thermodynamic stability, and spontaneous formation. Polymerization of microemulsified monomers has many advantages over traditional emulsion polymerization. Microemulsions are normally transparent to translucent so that they are particularly suitable for photochemical reactions, while emulsions are turbid and opaque. Also, the structural diversity of microemulsions (droplets and bicontinuous) is set by thermodynamics, and rapid polymerization may be able to capture some of the original structure. In addition, microemulsion polymerization enables production of stable, monodisperse microlatexes containing colloidal particles smaller than those produced from classical emulsion polymerization processes. Smaller particle size improves the ability to form coatings without microcracking. The increased surface area improves particle fusion during molding operations.
Emulsion polymerization, as opposed to microemulsion polymerization, of dissolved gaseous tetrafluoroethylene (PTFE) or its copolymers is a known process. Aqueous colloidal dispersions of PTFE or its copolymers can be prepared in a pressure reactor by placing the gaseous monomer, or a mixture of monomers in contact with an aqueous solution containing at least one surfactant which generally is a fluorinated surfactant, possibly a buffer for keeping the medium at a given pH, and an initiator which is capable of forming free radicals at the polymerization temperature. The free radical initiators can be water soluble peroxides, or alkaline or ammonium persulfates. Persulfate can be used alone if the polymerization temperature is above approximately 50.degree. C., or in association with a reducing agent such as ferrous salt, silver nitrate, or sodium bisulfate if the polymerization temperature is approximately between 5 to 55.degree. C., as described in the U.S. Pat. No. 4,384,092.
The gaseous monomer molecules in the foregoing process enter the aqueous liquid and react to form polymer without first forming a distinct liquid phase. Thus the polymer particles are large particles suspended in the aqueous mixture; and the process is not a true liquid-in-liquid emulsion polymerization. The process is sometimes referred to as dispersion polymerization.
Additives have been used in attempts to alter the polymerization processes and products thereof. For example, in U.S. Pat. 3,721,638, a perfluorinated ether ketone is taught as being added to an aqueous phase polymerization system for polymerizing tetrafluoroethylene, but the initial product is in the form of an aqueous gel.
Attempts have been made to prepare tetrafluoroethylene copolymers in aqueous dispersion systems. For example, EP 0612770 teaches the copolymerization of TFE and fluoroalkyl perfluorovinyl ethers in an aqueous system containing methylene chloride to obtain dispersion copolymer particles of an average of less than 50 nm in size.
U.S. Pat. No. 4,864,006 describes the polymerization of TFE and hexafluoropropylene (HFP) to make a copolymer in an aqueous microemulsion containing a perfluoropolyether in which the resulting copolymer particles have a size ranging from 0.041 to 0.070 micrometer.
Microemulsion polymerization operates by a different mechanism than emulsion polymerization. It involves polymerization of liquid monomer rather than gaseous monomers. Because the polymerization involves polymerizates of unusually small cells of liquid monomer, the resulting polymer particles are unusually small. However, polymerization of liquid TFE is not usually practiced, because of the potential hazards of handling liquid TFE.
It is desirable to provide a process for polymerizing TFE to produce homopolymer dispersions in which the particle size of the polymer particles is very small. Microemulsion polymerization systems would be useful in reaching this goal if a means could be found for adapting TFE to polymerization in an aqueous microemulsion system. Such a TFE polymerization system would result in small particles.